Much of the misunderstanding, I imagine, is that people who have never worked in a lab, such as these TSA employees, think that 'swabbed positive' means 'touched explosives'.
These quick tests just use a reagent that reacts with a part of a molecule. In the case of an explosives test, I imagine it responds to molecules with a -NO2 group. That could be TNT or any of a thousand perfectly innocent, harmless substances. A test that tests for the whole molecule, with a smaller (but still non zero) chance of a false positive, would be too expensive and take too long for a situation like this.
If you're just testing average people, the chance of a false positive is orders of magnitude more likely than a real positive.
These quick tests just use a reagent that reacts with a part of a molecule. In the case of an explosives test, I imagine it responds to molecules with a -NO2 group. That could be TNT or any of a thousand perfectly innocent, harmless substances. A test that tests for the whole molecule, with a smaller (but still non zero) chance of a false positive, would be too expensive and take too long for a situation like this.
If you're just testing average people, the chance of a false positive is orders of magnitude more likely than a real positive.